What I do is cut around the bottom most 3 heat seals... I cut in the back, kind of slicing in behind it between the two halves, then I just cut the "ends" of the back of the heatseal off (this is hard to explain, really). By cutting the 3 bottom heat seals on the back then like this, you can just lift the back half of the clamshell up and slide the figure out as well as easily nab the tab of course.
It's not markless and you can obviously see the cuts to the bottom 3 heat seals, but they're small generally, and I keep them to the back of the clam shell of course so when the CS is hanging on display you can't see any cuts.
It is just my way of keeping things displayable though, and I've found no perfect way of getting heat sealed clamshells apart unfortunately.